Although so-called radiography employing silver halide photography is conventionally used to obtain radiographic images, there also has been developed a radiographic imaging method not using silver halide photographic material. Thus, an imaging method is disclosed, in which radiation that has been transmitted through an object is absorbed by phosphor, followed by exciting the phosphor with energy to cause a radiation energy accumulated in the phosphor to radiate in the form of fluorescence, and imaging is achieved by detecting the fluorescence.
Specifically, U.S. Pat. No. 3,859,527 discloses a radiation image conversion method, in which a panel comprising on a support a photostimulable phosphor layer is employed using either or both visible light and infrared rays as the stimulating energy. There have been developed radiation image conversion methods using a photostimulable phosphor(hereinafter, also denoted simply as a stimulable phosphor) exhibiting enhanced luminance and high sensitivity, including, for example, a radiation image conversion method employing BaFX:Eu2+ type phosphor (X: Cl, Br, I), as described in JP-A No. 59-75200 (hereinafter, the term, JP-A refers to Japanese Patent Application Publication); a radiation image conversion method employing an alkali halide phosphor, as described in JP-A No. 61-72087; a radiation image conversion method employing an alkali halide phosphor containing, as co-activators, Tl+ and metals such as Ce3+, Sm3+, Eu3+, Y3+, Ag+, Mg2+, Pb2+, or In3+, as described in JP-A Nos. 61-73786 and 61-73787.
Recently, a radiation image conversion panel exhibiting further enhanced sharpness has been desired in the field of diagnostic image analysis. Of these, an attempt in controlling the form of stimulable phosphor grains to enhance sensitivity and sharpness was made as a means for improving sharpness of radiographic images. For example, JP-A No. 61-142497 discloses a method of using a stimulable phosphor layer comprising a fine columnar block which has been formed by sedimentation of a stimulable phosphor on a support having fine protruded patterns; JP-A 62-39737 discloses a method of using a radiation image conversion panel having a stimulable phosphor layer having a pseudo-columnar form which has been formed by producing cracks on the layer surface side; JP-A 62-110200 proposes a method in which a stimulable phosphor layer having voids is formed by vapor deposition onto the upper surface of a support, followed by growing voids by subjecting a heating treatment to produce cracks.
JP-A No. 2-58000 proposed a radiation image conversion panel having a stimulable phosphor layer, in which long and thin columnar crystals were formed with an incline at a given angle toward the direction normal to the support.
In the foregoing attempts to control the form of a stimulable phosphor layer, it was intended to enhance image quality by allowing the phosphor layer to have a columnar crystal structure. It was supposed that the columnar form prevented traverse diffusion of stimulated emission light (or photo-stimulated luminescence), i.e., the light reached the support surface with repeating reflection at the interface of cracks (or columnar crystals), thereby leading to markedly enhanced sharpness of images formed by the stimulated luminescence.
Recently, a radiation image conversion panel using a stimulable phosphor containing an alkali halide such as CsBr as a basic substance and Eu as an activator, and the use of activator Eu leading to enhanced X-ray conversion efficiency, which has formerly not been achieved. Introduction of Eu as an activator necessitates to allow Eu to be homogeneously diffused in a crystal, in a di-valent form. However, stability of homogeneous diffusion of Eu within a basic crystal substance under vacuum and prevention of oxidation to tri-valent Eu became problems with preparation of a detector. Such problems have not yet been overcome in the market.
The use of rare earth elements as an activator achieved particularly enhanced X-ray conversion efficiency but resulted in problems in that formation of a homogeneous layer under vacuum was difficult due to vapor pressure characteristics; when a stimulable phosphor layer was formed through vapor phase growth (vapor deposition), there were often conducted heating treatments, such as heating raw material, heating a substrate (or support) during vacuum deposition and annealing (for relaxation of substrate strain) after forming the layer, so that the existing state of the activator was varied, producing problems such as homogeneous layer formation.
Accordingly, improvements were desired with respect to luminance, sharpness and homogeneity of the activator contained in the phosphor layer, as required as a radiation image conversion panel on the market.